Add 32-bit compat for ioctls that take struct ifgroupreq.

Use an accessor to access ifgr_group and ifgr_groups.

Use an macro CASE_IOC_IFGROUPREQ(cmd) in place of case statements such
as "case SIOCAIFGROUP:". This avoids poluting the switch statements
with large numbers of #ifdefs.

Reviewed by: kib
Obtained from: CheriBSD
MFC after: 1 week
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D14960